The idea of documenting the advances achieved in the ﬁeld of light water reactor severe accident safety research in the form of a book originated during the development of the research program for the Severe Accident Research Network of Excellence (SARNET), coordinated by IRSN France, that was started in March 2004, under the auspices of the Sixth Framework Research Program of the European Commission. After some discussion, it was decided that the book should be a textbook for students and young researchers in the ﬁeld and not a handbook. It was also decided that the book should not be a compendium of all the research in the ﬁeld, but rather should be written to impart understanding and knowledge about the complex physics of severe accidents. The physics of severe accidents involves several disciplines, including probability theory, neutron physics, thermal hydraulics, high-temperature material science, chemistry, and structural mechanics. Thus, imparting understanding and knowledge of severe accidents is not a simple task. It is nonetheless an important task in reactor safety, since a severe accident is the only source of risk to the public from an operating light water reactor power plant. Preventing and managing the consequences of a severe accident, which is the main goal of severe accident research, contributes greatly to reducing the public risk of nuclear power.

The importance of severe accident research was recognized by the EURATOM Part of the Framework Research Programs of the European Commission. The Framework Program No. 4 was totally focused on severe accident (SA) research. The European Commission is continuing its support of SA research and, through its support of SARNET, is encouraging the focus of national research efforts on an integrated European program of this research. This book, Nuclear Safety in Light Water Reactors: Severe Accident Phenomenology, describes the results obtained from these research programs, conducted over the last 15 years in Europe. The book also contains the results of SA research conducted over the years in the United States, Japan, Korea, Russia, and other countries. The research conducted in the United States, in particular, was the forerunner of the research that has been conducted in Europe. The knowledge gained in the U.S. research forms a very important base for SA research conducted throughout the world. The book, therefore, documents the data, phenomenology, and methodology developed for the description of severe accidents in all countries.

This book is a joint effort since it is a product of SARNET, a network. It was conceived as a pedagogical effort, however, written by acknowledged experts in the different areas of the SA ﬁeld. It was coordinated, chapter by chapter, by different experts and ﬁnally compiled and edited by the undersigned.

Chapter 1 of the book provides a historical review of the whole ﬁeld of reactor safety, with short introductions on the various severe accident phenomenological topics. It also attempts to provide an insight into the logic of advancements in rector safety since the birth of nuclear energy. The most recent tragic event at Fukushima is also brieﬂy described in Chapter 1, based on the information gained as of the end of June, 10, 2011. This description may need corrections as more complete information about these severe accidents at Fukushima emerges in time. Chapter 1 also brieﬂy deals with the advances in mitigating severe accidents achieved in the designs of some of the new (GEN III+) LWRs.

The remaining contents of the book follow the severe accident scenario, starting with the loss of cooling of the decay-heated core. The resulting core heat-up, core degradation, hydrogen production, core melting, accumulation of melt in lower head, failure of the lower head, hydrogen combustion, steam explosion, molten corium– concrete interactions (MCCI), ﬁssion product release, transport in the primary system, containment, and the like, are the subjects treated in the book.

A question that arose early in deliberations on the contents of the book concerned the maturity of the SA ﬁeld. We believe that nearly all the knowledge gained through SA research, and described in the book, is mature enough and will stand the test of time. It should also be stated that severe accident is still an active ﬁeld of research, and some issues remain open; notably, the knowledge base still has uncertainties, and more research (experimentation, modeling, validation, etc.) is needed, and indeed is being performed. Even for these issues, however, a sufﬁcient knowledge base has already been acquired to document these areas of uncertainties.

The book is a product of the efforts and dedication of the contributors listed with the text in the book. Very substantial efforts were involved in editing the various contributions and organizing the book. We hope that it will be a worthwhile book for the education of nuclear engineering students and a reference text for the young researchers who want to work, or are already engaged, in the ﬁeld of LWR severe accident safety.

Last, but not the least, we the contributors (authors) and the Editor wish to acknowledge with thanks, the steadfast and continous support of M. Michel Hugon, the EU Program Manager for the SARNET Network of Excellence. The Editor, also wishes to acknowledge with thanks, the able assistance of Dr. Van Dorsselaere in the “ﬁnal-edit” of the book.